Orthogonal frequency-division multiplexing (OFDM) has become a key encoding method used by many communications technologies ranging from wireline to wireless technologies. In fact, OFDM use is pervasive, being employed by many technologies including, but not limited to, wired communications, such as Digital Subscriber Loop (DSL), Asymmetric DSL (ADSL) and Very-high-bit-rate DSL (VDSL) broadband access technology over Plain Old Telephone Service (POTS) copper wiring, Digital Video Broadcasting (DVB), Power Line Communications (PLC), ITU-T G.hn for home wiring LANs, telephone modems, DOCSIS—Data Over Cable System Interface Specification for broadband delivery, MoCA—Multimedia Over Coax Alliance home networking, and wireless communications including IEEE 802.11 (e.g. Wi-Fi), HIPERLAN, Digital TV, Personal Area Networks (PAN), and Ultra Wideband Networks (UWB).
OFDM and its multiple access variant OFDMA continue to find increasing applications, for example in 3rd Generation Partnership Project (3GPP)-based wireless networks such as Long Term Evolution (LTE) and Evolved Universal Mobile Telecommunications System Terrestrial Radio Access (E-UTRA) networks, but also in IEEE-based networks such as Mobile Broadband Wireless Access (MBWA, also referred to as IEEE 802.20). Next Generation mobile networks are planning to use OFDM as the platform for this new and exciting product evolution, and even the Wireless Gigabit Alliance (WiGig) plans to use OFDM in the 60 GHz frequency band to enable conference room cell sizes to achieve 100 Gigabits per second (Gbps) data rates.
Although these technologies are all based on OFDM, they have significant differences in their technology implementations. OFDM is a digital modulation technique that uses frequency division multiplexing to create multiple orthogonal sub-carriers to carry parallel data streams. Sub-carriers are modulated using conventional modulation schemes such as Binary Phase Shift Keying (BPSK) or Quadrature Amplitude Modulation (QAM) with defined symbol rates enabling multiple parallel data streams to be carried.
The detailed implementations for these various technologies are all quite different, largely driven by channel limitations or restrictions, and desired operational features. For example, 802.11a Wi-Fi employs short 3.2 microsecond (μs) symbols (with 0.4 or 0.8 μs for the cyclic prefix), and 52 carriers spaced at 312.5 kHz to create a high speed data channel capable of withstanding the low dispersion experienced in short reach indoor channels which Wi-Fi Access Points (APs) typically address, while LTE typically employs longer 66.7 μs symbols (or 71.4 μs with the cyclic prefix) with 15 kHz spaced subcarriers to address significant inter-symbol interference issues typical of long reach outdoor cellular channels.
Thus, implementations may differ by symbol time and sub-carrier spacing, but also by many other physical layer parameters including the number of sub-carriers, channel spacing, Fast Fourier Transform (FFT) size, number and operation of pilot tones, convolutional codes employed, Forward Error Correction (FEC) design, sub-carrier modulation schemes, time-interleaving, equalizer operation, and Multiple-Input Multiple-Output (MIMO) operation to name a few. Moreover, with the Medium Access Control (MAC) layer defining how the OFDM based physical medium is used by higher layer applications, OFDM designs are inherently complex and specific to a particular OFDM technology. As a result, the implementation of interworking functions with other OFDM based technologies has proven to be very difficult.
Nevertheless, with the explosion of wireless technologies in unlicensed spectrum such as the Unlicensed National Information Interchange (U-NII) bands managed by the Federal Communications Commission (FCC) in the United States, there is a desire to see upcoming technologies such as LTE work together to share this spectrum fairly with incumbents such as 802.11 (e.g. Wi-Fi) the dominant technology, and provide a positive end user experience. 3GPP and some to come 5G licensed networks will shortly begin trials to offer services in unlicensed bands. License Assisted Access for Long Term Evolution (LAA-LTE or LAA), as the first example, has recently demonstrated cabled operation at Mobile World Congress in March of 2015 using the 5 GHz band. Product rollouts are planned in 2016 and 2017. However, concerns over interoperability of these different technologies have been raised, driven by expectations of wide scale deployment of LTE radios into the unlicensed bands.
Since the FCC first made available spectrum in the 5 GHz band for U-NII operation in 1997, an etiquette protocol for medium access was developed for Wi-Fi systems which can be generalized into three rules:                1. Listen Before Talk (LBT)—Do not use the channel if Radio Frequency (RF) energy above a threshold (e.g. −62 dBm) is detected,        2. Carrier Sense—Do not use the channel if a Wi-Fi preamble (or related pilot tones) is detected, and        3. Virtual Carrier Sense—Do not use the channel while a channel busy timer (e.g. the Network Allocation Vector (NAV)) is counting down to zero.        
In Wi-Fi systems, the Clear Channel Assessment (CCA) function employs these simple etiquette rules to ensure that many Wi-Fi devices can share the same unlicensed channel fairly, and avoid transmission collisions which may have deleterious effects to both the interferer and interferee.
With the introduction of new 3GPP-based cellular technologies such as LTE and soon to be 5G into the unlicensed bands, an expanded etiquette will be required. Wi-Fi, as the main incumbent technology, has a defined etiquette. However, Wi-Fi does not address the complexities and requirements of 3GPP systems. Although they both use OFDM and both support a number of common features at the physical layer, 3GPP and Wi-Fi are fundamentally different.
One of the most fundamental differences is synchronization. Wi-Fi operates asynchronously by applying the etiquette rules and sending/receiving packets when the medium is free. In contrast, 3GPP operates synchronously and employs advanced scheduling algorithms to maximize channel utilization, and therefore is not burdened with etiquette rules. As a result, 3GPP is able to carry higher traffic loads efficiently i.e. in a way that maximizes the use of the valuable frequency channel resources.
Because of this and other notable differences in OFDM implementation, 3GPP-based technologies are not currently designed to support a sharing etiquette, such as that which Wi-Fi supports.
Different possible solutions have been proposed so far claiming to have the potential for improving fair sharing. One such proposal includes implementing a power-based LBT detect threshold. With this proposal, the LTE radio would monitor energy on the channel and consider it free if the received signal strength indication (e.g. RSSI) is lower than that threshold. However, this proposal does not address the variability of cell sizes due to unlicensed band interference. Also, in some implementations, the threshold is fairly large (−62 dBm) and limits the cell size. Depending on the channel conditions, there is no guarantee that the LTE radio will detect a transmitted signal above the threshold and this ultimately may result in a higher collision count and lower throughput.
Other proposals contemplate using a Wi-Fi receiver in the LTE radio to monitor and detect Wi-Fi pilot tones and/or transport LTE data using Wi-Fi packets. However, these proposals would involve significant hardware and software development and would not be backward compatible to existing LTE radios currently deployed. Moreover, the proposals combine transceivers which are fundamentally different to try and create a coordinated design. In doing so, it mixes the performance and regulatory aspects of two separate and independent radio transceivers, making the solution extremely complex to design, verify, and have certified since all of the key design parameters such as power control, AGC, power spectral density, PAR reduction techniques, and PA linearization techniques such as digital pre-distortion are now operating on two separate PHY devices.
Accordingly, to address some or all of the drawbacks noted above, there is a need for improved methods and systems to facilitate co-existence in shared spectrum.